The present invention relates to radio frequency filters of the type comprising a block of dielectric matter and an insulation sheet. The surfaces of the block comprise a top surface and bottom surface on the opposite sides, two opposite side surfaces limited to said surfaces, and the opposite two end surfaces. From the top surface of the block to the bottom surface at least two holes extend, coated with a conductive material. At least the main part of the surface of the body, with the exception of one side surface, has been coated with a conductive layer, whereby a transmission line resonator is produced for each hole. The insulation sheet has been attached against the uncoated side surface, the surface of which not facing the block is coated with a conductive layer.
Known in the art are such dielectric, usually ceramic filters which have been coated all over, with the exception of the top surface, with an electrically conductive material. When the coating of a coated hole is connected with the coating of the bottom surface, the hole has been short circuited at that point. Since the top surface, at least in the vicinity of the hole, is uncoated, the hole is open at this end. The structure forms now a quarter-wave transmission line resonator. When conducting an electromagnetic wave into the structure, at a given frequency, i.e. at the resonant frequency, a standing wave in the direction of the hole is produced. The maximum of the capacitive field thereof is placed at the open end of the hole, whereas the maximum of the inductive field is placed at the short circuited hole. If on the uncoated top surface various conductive patterns are placed, both the resonant frequency of an individual resonator and the coupling between the resonators can be affected. By positioning a conductive pad adjacent to the open end of the furthermost resonators of the block, and insulated from the coating of the side of the block, a signal can be carried into the resonator by being capacitively coupled with the resonator, and out therefrom likewise by capacitive coupling. Since between the coating of the open top end of the resonator and the coating of the top side edge of the ceramic block is a given capacitance value, said capacitance can be changed by adding some coating to the proximity of the top side hole,said coating being in connection with the coating of the side, or by adding some coating on the top side in connection with the coating of the hole. This is one manner in which the resonant frequency is affected. With the aid of the conductive patterns, capacitors and transmission lines may further be arranged on the top surface also between the resonators, and so, the coupling between the resonators can be affected.
The inductive coupling between the resonators can be affected by handling the ceramic block, e.g. by boring holes therein or by removing otherwise some of the matter.
Positioning conductive patterns on the top surface of the ceramic block is, however, very difficult because the surface area available is very small, so that even minimal defects in the accuracy in positioning the conductor patterns greatly affect the electrical properties of the filter. In addition, by placing the conductive patterns merely on the top surface, only the capacitive field can be affected, and the couplings are therefore capacitive.
A critical improvement in said generally used method is disclosed in the patent application EP-0 401 839 of the present applicant, Turunen et al., whose U.S. counterpart issued as U.S. Pat. No. 5,103,197. In the filter described therein the electrical properties of the filter can be affected in a wide range in that the side surface is substantially uncoated, and the conductive patterns and the coupling wires have been placed on said side surface of the filter block. Not only is the surface area available much larger for positioning the conductive patterns than in placing them on the top surface, but also the inductive coupling between the resonators can be affected. It is true that the inductive field is largest in the short circuited lower end of the resonator. Positioning a conductive pattern on a side surface allows making the coupling between the resonators capacitive, inductive and capacitively-inductive in one and same filter block. Also the coupling to the filter can be performed inductively, capacitively and as a combination thereof. The electrical properties of the filter are not so sensitive to minor variations in positioning the conductive patterns on a block side as they are when the patterns are positioned on the top surface with a small surface area. According to the EP application, the side on which the conductive patterns are located, is finally coated with a metallic cover. Said filter construction allows considerable freedom for the filter designer, and in practice, by using merely a few standard-sized filter blocks, it is possible, by varying the bandwidth and the mean frequency of the resonators, that is, by using different conductive patterns, filters of different types can be constructed.
In the EP application, also another embodiment is described. As taught thereby, the side surface of the block is also substantially uncoated. An insulation sheet is placed against the side surface, the surface not facing said surface of the block as well as the edges of the sheet have been coated. The coating is electrically in connection with the coating of the block. The conductive patterns have therefore been placed on the surface of said insulation sheet positioned against the uncoated side surface of the ceramic block. This is preferable particularly when the insulation sheet is part of the circuit board whereon also the rest of the components required in the circuit are placed. Such discrete components can be, e.g. coils and surface mounted resistors. Since it is difficult to obtain high inductance values with the conductive patterns, discrete coils are needed in a variety of filters, such as band stop filters between different resonators through which a signal passes from one resonator to another. Said discrete components are placed on the part of the insulation sheet which extends across the side surface of the filter block. Carrying a signal into a filter as well as therefrom can be performed with strip conductors via said crossing part.
The construction according to the EP application mentioned above and particularly the embodiment in which the conductive patterns and coupling parts are placed on the insulation sheet positioned against the side surface contain serious drawbacks in spite of certain advantages. The first one is the requirement concerning the straightness of the surfaces. Both the side surface of the block and the insulation sheet placed thereagainst are required to be extremely plain so that no air gaps are left therebetween when the surfaces are placed one against the other. The second one concerns the requirement set on adjusting the insulation sheet. When the patterns are located on the insulation sheet and they must be positioned precisely on a given pad against the side surface of the block, even minor divergence in positioning generate variations in the electrical properties in the finished products, which may exceed the permitted tolerances.